Basically you are right. However, classe in portu relicta is an ablative absolute which - to me at least - seems to indicate merely circumstances of lesser importance. Aeneas did not leave the fleet in the harbour in order to traverse the city. Of course, he has to do so to be able to traverse the city (theoretically he might raze the town and draw the ships on shore and all along to the dwelling place of Helenus . But the traversal is only a means. Therefore I would rather say: Aeneas classem in portu reliquit et ad urbem progrediebatur ut cum Heleno loqueretur...

Carolus, thanks for your guidance. I thought progredi means to go to / towards a place rather than cross it and that, with acc. it would mean going towards the city....but what you say seems to make sense here...

Surely it does mean "go to / towards a place"; except for that, I agree with Carolus Raeticus: "Since Aeneas wanted to hear it from Helenus himself, he set forth towards the city (after having left the fleet in the harbour)."